A typebar is an 'arm' inside a typewriter

Typewriter

A typewriter is a mechanical or electromechanical device with keys that, when pressed, cause characters to be printed on a medium, usually paper. Typically one character is printed per keypress, and the machine prints the characters by making ink impressions of type elements similar to the pieces...

 with a character on the end of it. There are generally two characters per typebar, one which will be printed if the corresponding key is struck by itself, the other of which will be printed if the corresponding key is struck while the shift key

Shift key

The shift key is a modifier key on a keyboard, used to type capital letters and other alternate "upper" characters. There are typically two shift keys, on the left and right sides of the row below the home row...

 is depressed. To have letters typed in exact location, the typebar is guided all its way to the ribbon through its narrow gap in segment.

In early typewriters in the 1860s, the typebars would often jam when two or more keys were depressed simultaneously (see picture). Such jamming led to the creation of the QWERTY

QWERTY

QWERTY is the most common modern-day keyboard layout. The name comes from the first six letters appearing in the topleft letter row of the keyboard, read left to right: Q-W-E-R-T-Y. The QWERTY design is based on a layout created for the Sholes and Glidden typewriter and sold to Remington in the...

 keyboard in 1873. Mechanisms to avoid such jamming had been created by 1879, but the keyboard layout remained as the legacy of the early problem.